Isocyanates are required by the US military as curing agents for hydroxyl-terminated polymers, which are used as binders in many cast-cured solid propellants and plastic-bonded explosives. Several factors are considered in selecting the best isocyanate for the desired application: processability and mechanical properties, pot life and cure rate, toxicity, tendency to hydrolyze, stability and aging, and availability.
3-Isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate, commonly known as isophorone diisocyanate (IPDI), excels in most of these categories. Its desirable pot life and cure rate are due to two isocyanate groups with different reactivities, one hindered and the other unhindered. Generally, isocyanates are extremely toxic, but IPDI's lower volatility makes it easier and safer to handle than other candidates. The mechanical and chemical properties imparted by use of IPDI are also good.